Binary coating of refractory metals



Sept. 28, 1954 E WAlNER 2,690,409

BINARY COATING F REFRACTORY METALS Filed July 8, 1949 Patented Sept. 28,1954 BINARY COATING OF REFRACTORY METALS Eugene Waner, Cleveland, Ohio,assignor to Thompson Products, Inc., Cleveland, Ohio, a

corporation of Ohio Application July 8, 1949, Serial No. 103,632

(Cl. 14S-31.5)

4 Claims.

The present invention relates to a method of coating refractory metalarticles to enhance their resistance to high temperature and corrosiveatmospheres. The present invention specifically relates to themanufacture of coated refractory metal turbine buckets for use in jetturbines and the like.

Turbo-jet engines or the like are usually provided with an axial owturbine operated by exhaust gases which drive a blower furnishing air tothe burners. Such turbines operate at extremely high temperatures, andone of the major difiiculties encountered in the manufacture of jetturbines has been the provision of suitable material for bucket bladeswhich can withstand the effect of such high temperatures. The turbinebucket will normally be exposed to temperatures in the range of from1600 to 2000 F. and the bucket must have sufiicient strength, toughness,creep resistance, and resistance to the corrosive atmosphere present toenable the bucket to operate efficiently without deformation orcorrosion.

In addition to turbine buckets, articles produced by the presentinvention may be employed under conditions of higher temperature andlower stress than exist in a gas turbine bucket. One such applicationoccurs in nozzle diaphragm vanes in gas turbines which must withstandvery severe conditions of temperature and thermal shock 'but at arelatively lower stress.

One refractory metal which exhibits excellent properties of strength,toughness and creep resistance at elevated temperatures is molybdenum.However, metallic molybdenum itself cannot be used. The trioxide ofmolybdenum, which is formed under the oxidizing conditions present inthe turbine, sublimes at a temperature of about 1463 F. at an extremelyrapid rate. This phenomenon gives rise to a characteristic smoking whenbodies of molybdenum are heated to tempera-tures above 1463" F.,resulting in the complete disappearance of the molybdenum within amatter of minutes. Another refractory metal which might be used forturbine bucket bodies is tungsten, even though it has a relatively highdensity.

To overcome this difficulty, I have herein provided a process forcoating refractory metal such as molybdenum to provide thereon a tough,corrosion-resistant coating impervious to the eect of oxygen and othergases.

It is then an object of the present invention to provide a method forcoating refractory metals such as molybdenum to provide an extremelycorrosion resistant surface thereon.

Another object of the present invention is to provide a method forcoating refractory metals which yields a rm bond to the refractorysurface and makes it impervious to the effects of operation underconditions of high stress and high temperatures.

A further object of the invention is to provide a coated molybdenumarticle, such as a turbine bucket, capable of operation within a turbineengine for extended periods of time without deteroration.

In the process of the present invention, the refractory metal is coatedwith a metal selected from the group consisting of silicon, aluminum,and zirconium and the coating is subsequentlyreacted with anotherelement to produce a binary coating. The coating and reaction steps maybe carried out concurrently, or the refractory metal may be given aprimary coat of silicon, aluminum or zirconium and subsequently reactedwith the second element, which is preferably selected from the groupconsisting of the elements in groups III, IV or V of the periodic table.

The element which is to be reacted with the primary coating is one whoseionic size is close enough to the ionic size of the primary metalcoating to allow the elements to be mutually soluble with each other inthe solid state, and thus increase the rate of intermetallic compoundformation. Such intermetallic compounds per se or the compounds formedwith the base metal then have a size which approximates the atomicspacing in the lattice of the base metal. The primary coating inherentlyleaves microscopic voids, tunnels or weak spots in the surface of thearticle, thus decreasing its ability to withstand corrosion. By reactingthe primary coating with a second element of the type menticned, it isbelieved that the voids in the atomic spacing of the single metal areiilled by the reaction with the second element, thus making the layermuch less permeable. The reaction with the second element dependsprimarily upon the spacing in the crystal lattice of the primarycoating. Where the primary coating is silicon, the secondary element maybe titanium, zirconium, boron, aluminum, nitrogen, or carbon. In thecase of aluminum, a primary coat of this metal may be further reactedwith zirconium, titanium, chromium, boron, tin, and nickel. Wherezirconium is used as the primary coat, the subsequent reaction may becarried out with aluminum, boron, carbon, silicon, titanium, andnitrogen.

The compounds resulting from the reaction of the second named elementswith the primary coating are complex intermetallic compounds Whichexhibit the property of forming an eX- tremely firm bond to the surfaceof the refractory metal. It is believed that this rm bond results fromthe closure of voids in the atomic lattice of 'the-base metal, but thepresent invention is in no way limited to the correctness of the giventheory.

The coating process of the present invention may be most convenientlycarried out in a vapor phase deposition system of the type described ina copending application Serial No. 98,272, led June 10, 1949 by myselfand Robert A. Kempe. In that application, there is described a Amethodfor coating refractory metals wherein a decomposable compound,preferably a halide, of the coating metal is carried into a reactionzone in a stream of hydrogen and therein decomposedto form a layer ofsubstantially pure metal on the surfaces of the refractory metal.Deposition of thecoating metal compound, as discussed in this previousapplication, vis-the result of several factors. Some of the compound isprobably decomposed vby the high temperatures,on the order of 1600 to2300 F. present in the coating Zone. Another portion of 'thedecomposable coating compound is reducedby the presence of the hydrogenatmosphere in which the compound is introduced. Another reaction which-occurs is the metathetical reaction between the coating metal compoundand the -molybdenum wherein the coating metal is deposited on themolybdenum with `the rformation of a volatile molybdenum compound inthe-exchange reaction.

The decom'posable compoundlemployed in the primary coating step ispreferably a halide, for example silicon tetrachloride, trichlor silane,silicon-tetrabromide, tribromo silane, silicon tetraiodide, aluminumchloride, valuminum bromide, aluminum iodide, vzirconium chloride,zirconium bromide, or zirconium iodide. vWhere the coating metalcompound exists `normally in the liquid state, vas is in the case-ofsilicon tetrachloride, the coating metal compound may be introduced intothe 'heated reaction AZone Vby merely passing a stream of hydrogen gasthrough a liquid pool of the compound so -that vapors thereof arecarried by the hydrogen into the reaction zone. Where the coating metalcompound `is normally solid, the hydrogen `gas vmay be passed `over aheated source ofthe c'ompound'in powder vform 'to carry it' into'th'ereaction zone.

lIn reacting the primary coating with the'second element, a decomposablecompound capable of yielding the second element is introduced intothereac'tion zone either prior to, during, or after the deposition ofthe primary coating. vTypical decomposable compounds which may be usedin this connection are titanium tetrachloride, titanium tetrabromide,chromium chloride, nickel chloride, boron chloride, and tinchloride.Where thepri'mary metallic 'coating is to be carburized, i. e., reactedwith carbon, a carburiz'ing gas is introduced into the reaction zone.Any common carburizing gas usuch as -natural gas, lcarbon monoxide,ethane, propane, `lnitane, and benzene maybe used, or the carburizingoperation may be carried out in carburizing packs, or in liquid baths.

Where the primary coating is to .be nitrided, i. e., reacted withnitrogen, ammonia gas Vis preferably introduced into the reactionchamber to react .with'the primaryscoating. In addition to 4 ammoniagas, various decomposable cyanides may be employed.

The reaction conditions in the coating Zone include temperatures from1600u to 2300 F. and preferably from 1700 to 2l00 F. and reaction timesranging from 4 to 24 hours depending upon the vpenetration required.Normally, the depth of the primary coating will -be on the'order ofabout 0.0003 to about 0.003 inch.

AWhere silicon is used as the primary coating metal, vthe ,outermostlayer is substantially pure silicon, the nextlayer is probably amolybdenumsiliccn compound having a relatively high silicon content,possibly MoSiZ while the innermost layer producedby "the coating processis essentially a molybdenum-'silicon compound having a high molybdenumconcentration, such as MozSi together with solid solutions of molybdenumand silicon. rhis type of structure is one whichis stable at hightemperatures, and provides an excellent intimate bond with the.-molybdenum base. The reaction with 'the Ysecond element of the typementioned above enhances'the properties of the coating by filling up themicroscopic voids and Weak spots, resultingin a substantial increase instability andresistance to high temperature oxidation.

VA further description of the present invention will .be made inconnection with the attached sheet of drawings in which:

Figure 1 is aow sheet showing in'generalthe various stages of thecoating process; and

Figure v2 is a drawing of a iphotomicrograph taken as a magnication of500K showing the crystal structure of a molybdenum article coated in.accordance with the `present invention.

As'shown on the drawings:

Reference numeralV I0 denotes a'supply of purging gas, which ispreferably an inert gas such as nitrogen, argon, neon,'he1ium or thelike which is passed .into apurication zone I I where moisture andVother contaminantsare removed. The purication zone II may consist of asupply `of liquid sulfuric `acid through whichthe purging gas 'isbubbled. v'The `purified gas is next introduced into a heated 'furnaceIZWhichsurrounds a Vfurnace tube I3, control of .the gas owing into thefurnace'tube being .controlled by `means of a valve I4.

Disposed Vwithin the Vfurnace tube I3 are a plurality of boats L5 Ywhichcarry 'a number of turbine buckets I6 or other article's composed of arefractory metal such as .-.molybdenum- These articles are `normallypreshaped into their 'desired form, and Worked at temperatures below therecrystallization temperature Aof the metal, to enhance its physicalproperties. The temperature of the furnace I2 is regulated between 1600and 2?00o F., with .1700to 2100o F. being a preferred range.

A supply of hydrogen gas I1 is provided for introduction intothefurnacetube I3. VPrior to its introduction into the furnace tube I3,the hydrogen gas is dehydrated andpurined by means of various desiccantssuch `asjfor example, packed columns of silica gel, calcium chloride,'or liquid sulfuric acid in a puriiication stage I8. The owofthehydrogen gas into the furnacetube I3 .is vcontrolled by `'means Vof avalve vI9. .A secondfsource of hydrogen gas 20 and its purication stage2| is also providedto act asa lcarryingmedium'for the coating compounds.`A source of the primary coating-compound A, normally adecomposablehalide, kis maintained in Va zone 22. A source of acompound, -B, of .the Ysecond `reactive element which may be one of themetals given previously, or a carburizing or nitriding gas is indicatedat zone 23. Flow into the zones 22 and 23 is regulated by means of therespective valves 2li and 25 while flow of hydrogen gas containing thecoating compounds is regulated by means of the valves 23 and 21 at theexit of stages 22 and 23. In this arrangement, either of the coatingcompounds may be introduced separately into the reaction zone or theintroduction `of both compounds may be made simultaneously.

Initially, the furnace tube I3 is purged by means of the purging gas I9to rid the furnace chamber of moisture and oxygen or other undesirablecontaminants. Thereupon, the hydrogen and the coating metal compoundwhich it carries is introduced into the furnace tube i3, Where itcontacts the molybdenum article I6 for periods of time ranging from 4 to24 hours. Excess hydrogen is vented from the furnace tube by means oftube 23. The molybdenum articles I6 are maintained in the furnace tubesI3 until a coating having a thickness of approximately .0093 to .093inch is produced on the article.

In Figure 2, there is shown a drawing of a photomicrograph oi amolybdenum turbine bucket produced by simultaneous vapor deposition ofsilicon and titanium for a period of eight hours at a temperature of2000 F. As illustrated in that drawing, the body of the article consistsof rather large crystals of molybdenum 29 having an overlying layer ofsilicon 30. Immediately above the silicon layer 30 is a layer 3l ofindeterminate composition, which is probably a complex mixture ofmolybdenum, titanium and silicon in the form of various intermetalliccompounds. The uppermost layer 32 is a mixture of titanium and siliconcompounds of these two elements. This structure has been found capableof withstanding operation in a gas turbine operating at temperaturesestimated at 1600" to 1800 F. for periods in excess of 100 hours withoutapparent deterioration.

From the foregoing, it will be appreciated that I have herein provided aprocess for coating refractory metals which are in themselves incapableof withstanding the corrosive effect encountered in the operation of agas turbine.

The coatings produced in accordance with this invention are integrallybonded to the base metal and cannot be stripped mechanically from thebody metal, as is the case of coatings applied by electroplating ordipping. Further, the coatings are inert with respect to the molybdenumbase metal and show no evidences of reaction with the base metal afterthe original deposition.

The vapor phase deposition processes described herein have completethrowing power i. e., a uniform coating can be deposited over the entiresurface of the article regardless of corners, grooves or otherirregularities in the surface of the article. This is not true of othertypes of coating procedures.

It will be understood that modifications and variations may be effectedwithout departing from the scope of the novel concepts of the presentinvention.

I claim as my invention:

1. The method of providing a corrosion resistant surface on a molybdenumarticle which comprises depositing a layer of silicon on said article,and nitriding the silicon layer.

2. A molybdenum body having a corrosion resistant outer layer ofnitrided silicon thereon.

3. The method of providing a molybdenum body with a corrosion resistantcoating, which comprises depositing on the surface of said body a layerconsisting essentially of an element selected from the group consistingof silicon and zirconium, and reacting the selected element in saidlayer with a different element selected from the group consisting ofsilicon, zirconium, titanium, boron, aluminum and nitrogen to form abinary coating on said body, said coating being integrally bonded to themolybdenum` body.

4. A molybdenum body having an outer corrosion resistant coating thereonconsisting essentially of the reaction product of an element selectedfrom the group consisting of silicon and zirconium with a differentelement selected from the group consisting of silicon, zirconium,titanium, boron, aluminum and nitrogen, said coating being integrallybonded to the molybdenum body.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 1,551,333 Schroter et al. Aug. 25, 1925 1,929,252 Morris -1Oct. 3, 1933 2,096,924 Schwartzkopf Oct. 26, 1937 2,235,504 Rennie Mar.18, 1941 2,294,562 Kingston Sept. 1, 1942 FOREIGN PATENTS Number CountryDate 842,981 France June 22, 1939 OTHER REFERENCES Metals Handbook, 1939edition, pages 1054, 1055, 1074, 1090. Published in 1939 by the AmericanSociety for Metals.

1. THE METHOD OF PROVIDING A CORROSION RESISTANT SURFACE ON A MOLYBDENUMARTICLE WHICH COMPRISES DEPOSITING A LAYER OF SILICON ON SAID ARTICLE,AND NITRIDING THE SILICON LAYER.